Various types of marine cargo transfer devices have heretofore been proposed for loading a marine vessel, either from another vessel or from a shore facility, with fuel oil, crude oil, or other types of fluid cargo. Because of the shear weight and volume of the fluids being transferred, such marine loading equipment is necessarily large and difficult to handle. Particularly in transferring fluid cargoes at sea between two floating vessels, both of which are subject to motions of heave, roll, pitch, and yaw, as well as moving toward and away from each other under the action of wind and wave, severe strains can be placed on such cargo transfer equipment. It has been the practice, particularly in transferring fluids between two floating vessels, to utilize flexible hoses, the weight of the hoses and the material passing through the hoses being supported by cranes or other types of multiple pulley and cable arrangements from one or both of the vessels. Flexible hoses allow for relative motions between the two vessels. Such arrangements have not proved practical for transferring petroleum products that are liquefied at low temperatures, such as propane which must be maintained at temperatures below -50.degree. F., or a liquefied natural gas which must be maintained at cryogenic temperatures. Normal hose materials are not suitable for such low operating temperatures. While special flexible hoses have been provided for conducting very low temperature fluids, such hoses do not have the strength and durability to operate under the severe conditions encountered at sea.
Marine loading arms have been developed which utilize rigid pipe sections coupled by swivel joints, such as described, for example, in U.S. Pat. No. RE 25,855. By using rigid pipe sections, they can be made of materials, such as stainless steel, which retain their strength at low temperatures. However, such cargo transfer devices have not been suitable for transfer between vessels on the high seas, but have been restricted to dock side loading where the arms have been permanently mounted on the dock and remain relatively stationary. The only motion of the vessel is the change in draft with loading and unloading of the cargo. To make the loading arms more maneuverable, the arms have been counterbalanced by weights so that the arms remain substantially balanced in all operating positions.
However, such counterweighted arms are not easily adjustable for counterbalancing under variable loads, such as when the pipe sections are empty or filled with fluids of different specific gravaties. When transfering cryogenic fluids, for example, a heavy build-up of front end ice on the arms is common, adding greatly to the weight that must be counterbalanced.